mumble: Switch to upstream build system: ninja

ninja is a bit faster at building the software than make. My
measurements put it at about 20% faster.

# make
❯ nx hyperfine 'nix build .#mumble --rebuild' --warmup 1
--ignore-failure
Benchmark 1: nix build .#mumble --rebuild
  Time (mean ± σ):     73.287 s ±  2.462 s    [User: 0.679 s, System:
3.338 s]
  Range (min … max):   69.478 s … 77.686 s    10 runs

# ninja
❯ nx hyperfine 'nix build .#mumble --rebuild' --warmup 1
--ignore-failure
Benchmark 1: nix build .#mumble --rebuild
  Time (mean ± σ):     60.481 s ±  2.828 s    [User: 0.089 s, System:
1.173 s]
  Range (min … max):   57.687 s … 66.910 s    10 runs

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